A conventional thermocouple consists of a temperature-sensitive element to which electrical conductors are attached. The temperature-sensitive element produces a voltage or current which is related (e.g., proportional) to the ambient temperature around the element. Many such thermocouples include a outer jacket which encloses the temperature-sensitive element and conductors.
Since air is known to be a poor heat-conductor, any air space between the jacket and the temperature-sensitive element tends to increase the response time of the thermocouple to changes in ambient temperature. Thus, it is important that the jacket be in intimate contact with the temperature-sensitive element.
U.S. Pat. No. 845,413 to E. Haagn discloses an electric-resistance thermometer in which an outer glass sheath is melted onto a platinum wire which is wound around a glass rod. While no space appears to exist between the sheath and the wire in the Haagn device, this is achieved by a rather expensive and difficult process which involves evacuating the air within the sheath and heating it at a temperature as high as 1500.degree. C. More importantly, the Haagn device does not include a mechanism for relieving stress caused by differences in rates of thermal expansion or contraction between the wire and the glass.
Metal-jacketed thermocouples have long been commercially available. While a metal sheath offers protection to the temperature-sensitive element and conductors which it surrounds, the metal sheath itself is nonetheless vulnerable to the corrosive or oxidative substances present in detrimental atmospheric conditions or baths to which a thermocouple assembly is frequently subjected.
It is disclosed in U.S. Pat. No. 3,069,752 to Sherning that a metal-jacketed thermocouple is fitted into another protective sheath made of graphite to isolate the metal sheath from the ambience. There are several drawbacks associated with the use of a graphite sheath in a thermocouple assembly.
First, during assembly, a carbonaceous or key graphite paste has to be used to fill any voids, which inevitably exist between the thermocouple and the sheath, in order to shorten the response time of the thermocouple instrument thus assembled. The paste is subsequently dried out via a gradual and slow heating process. Thus, it is somewhat tedious to assemble such an instrument. Further, differentials in thermal expansion or contraction of the metal and graphite sheaths may cause one or both to rupture. In addition, graphite is a rather brittle and fragile material and thorough cleaning of a graphite-sheathed thermocouple may be difficult or uneconomical.
There exists a need, therefore, to provide a sheathed thermocouple instrument which provides a relatively rapid response to changes in ambient temperature, is easy to assemble and clean, is relatively sturdy and durable, is operative over a wide temperature range and which compensates for stress induced by rapid temperature change or extreme temperatures.